Nicholas James Adenhart was born in Silver Spring, Maryland, the only son of Janet and Jim Adenhart, a former United States Secret Service officer. His parents divorced and Janet later remarried Duane Gigeous, with whom she had a son named Henry, who is a pitcher at the University of Oregon. Adenhart played Halfway Little League Baseball for Gehr Construction and attended Springfield Middle School in Williamsport, Maryland. He pitched for the Hagerstown PONY League for six years, and was a member of the 1999 team that won the Maryland District 1 title.

After graduating from middle school, Adenhart attended Saint Maria Goretti High School in Hagerstown, Maryland. He played shortstop and outfield, in addition to pitching. While attending Saint Maria Goretti, Adenhart was a guard on the basketball team that won the Baltimore Catholic League championship.[6] At the age of 14, Adenhart joined the Oriolelanders, a showcase team composed of Maryland amateur players and sponsored by the Baltimore Orioles, where he would stay for four years. In 2003, at the age of 16, Adenhart pitched for the Youse’s Maryland Orioles, who went on to win the All American Amateur Baseball Association Tournament for that year.

Adenhart transferred to Williamsport High School after his sophomore year, where he gave up basketball to focus solely on baseball, as a pitcher. Scouts began closely following him when he was named the top junior prospect by Baseball America. Adenhart had a 6–0 record with a 1.04 earned run average during the regular season in his junior year, and as a result was named the Gatorade Player of the Year in Maryland. In a 1–0 loss during the playoff quarterfinal matchup, he threw a no-hitter and had 14 strikeouts. Entering his final high school season, Baseball America dubbed Adenhart the top high school prospect in the country. In his senior year, Adenhart threw a perfect game in his very first outing, striking out 15 of the 21 batters faced. Entering the final regular season game of his high school career, he had a 5–1 record, a 0.73 ERA and an average of 2.2 strikeouts per inning. During November of his senior year, Adenhart signed a letter of intent to play with the North Carolina Tar Heels baseball team at the University of North Carolina, although his chances of being drafted high continued to climb and the North Carolina coaching staff saw their chances of acquiring Adenhart to be slim.

In his final high school game, in front of two dozen scouts, Adenhart felt a pop in his elbow after throwing a curveball to the third batter. The injury, which abruptly ended his season, was a partial ligament tear in his elbow that required Tommy John surgery. Though he had originally been projected as a first-round draft pick, the injury caused his stock to plummet two weeks before the 2004 Major League Baseball Draft. He fell to the 413th overall pick in the 14th round, selected by the Anaheim Angels. Angels scout Dan Radcliff and director of scouting Eddie Bane convinced Adenhart to forego a scholarship offer from the University of North Carolina and signed him to a $710,000 bonus ($820,000 in 2010) on July 26, 2004. After having Dr. James Andrews perform the Tommy John surgery, Adenhart spent the next year rehabilitating his elbow at the Angels’ rehab facility in Tempe, Arizona. During that time, Adenhart also attended classes at Arizona State University.

Adenhart made his professional debut on June 25, 2005 with the Pioneer League’s Orem Owlz, the Angels’ Rookie League affiliate. In his lone appearance for the Owlz, he pitched six innings, allowed one unearned run, struck out seven, and earned the win. He spent the rest of the 2005 season with the Angels of the Arizona League that summer. In 13 games for the Angels, he had a 2–3 record, a 3.68 ERA, and 52 strikeouts. The following spring, at the age of 19, Adenhart was one of twelve pitchers who earned a non-roster invitation to the Angels’ big league camp. As the 2006 season began, he was also considered the Angels’ sixth best prospect and the 90th-best overall by Baseball America. Adenhart was assigned to the Cedar Rapids Kernels, the Angels’ Low-A affiliate, after spring training. He pitched well for the Kernels. In 16 games, Adenhart had 10 wins, a 1.95 ERA, and 99 strikeouts in 106 innings pitched. His performance earned him a starting assignment in the 2006 All-Star Futures Game on June 21 and a promotion to the Rancho Cucamonga Quakes, the organization’s High-A affiliate, soon after. He continued his efforts with the Quakes, winning five games and losing two in nine starts. Adenhart was a member of the United States Olympic Qualifying team in 2006, along with fellow Angels prospect Brandon Wood.

In 2007, Adenhart became a top-ranked prospect in the Angels organization. Baseball America ranked him as the 34th-best prospect in baseball and second in the Angels organization. He was called up to the Double-A Arkansas Travelers for which he played the 2007 season. In 26 appearances, he had a record of 10–8 with a 3.65 ERA. The following year, Adenhart was called up to the Triple-A Salt Lake Bees, where he spent the majority of the 2008 season. He was declared the 24th-best prospect in the majors that season. In the month of April, Adenhart had a 4–0 record with a 0.87 ERA and 21 strikeouts in 31 innings. As a result, he was brought up to the Angels’ major league roster, and was scheduled to make his debut on May 1, pitching on three days’ rest. Shortstop Maicer Izturis was placed on the disabled list when Adenhart was brought up.

Adenhart made his Major League debut as the starting pitcher against the Oakland Athletics at home in Angel Stadium of Anaheim. At the time, he was the youngest active-roster pitcher in the major leagues. In his debut, he gave up five earned runs and walked five in two innings, earning a no-decision in a 15–8 loss. Adenhart was disappointed in his first appearance, saying, “I let down the team first. You always want to go out and prove yourself to your teammates and your manager and your coaches. I was a disappointment to myself also.” His second start was against the Kansas City Royals, where he earned another no-decision, allowing three earned runs and striking out three over 4⅓ innings in a 5–3 win by the Angels. The reaction to his second start was more positive, with manager Mike Scioscia saying that “it was definitely a start in the right direction.” Adenhart earned his only career decision, a victory, on May 12, 2008, against the Chicago White Sox in Anaheim, giving up four earned runs in 5⅔ innings en route to a 10–7 Angels victory. After the win against the White Sox, Adenhart was sent back down to the Salt Lake Bees. He spent the rest of the 2008 season at Salt Lake, amassing a 9–13 record with a 5.76 ERA in 26 games.

Adenhart was declared the best prospect in the Angels organization going into 2009, and was ranked 68th overall on Baseball America’s 2009 Top 100 Prospects list, citing his 158 innings pitched per year over the past three seasons. Adenhart earned his spot in the Angels’ 2009 starting rotation during spring training. He appeared in six starts and had a 3–0 record with a 3.12 ERA over 26 innings pitched. He allowed only nine earned runs and five walks, while striking out 18. Adenhart opened the 2009 season as the third starter in the Angels’ rotation. In his season debut on April 8, 2009, he earned a no-decision, giving up seven hits and no runs while striking out five batters and walking three in six innings against the Oakland Athletics at Angel Stadium of Anaheim.

Shortly after midnight on April 9, 2009, Adenhart was involved in a car accident in Fullerton, California, just hours after being the starting pitcher in the previous night’s game. Police reported that an individual driving a red Toyota Sienna minivan ran a red light and broadsided a gray Mitsubishi Eclipse in which Adenhart was a passenger, sending it crashing into a telephone pole. Courtney Stewart and Henry Pearson, the driver and another passenger in the Mitsubishi respectively, were pronounced dead at the scene. Adenhart and Jon Wilhite, a third passenger in the Mitsubishi, were taken to University of California, Irvine Medical Center, where Adenhart died as a result of his injuries. Wilhite suffered internal decapitation and survived after undergoing five hours of surgery to reattach his skull to his spine six days after the accident.

The minivan driver fled the scene on foot, but was later arrested and identified as Andrew Thomas Gallo. On May 27, 2009, Gallo was indicted by the Orange County grand jury on three counts of murder, one count each of felony hit-and-run, driving under the influence and causing injury, and driving with a .08 percent blood alcohol or higher and causing injury and death. Gallo had a blood alcohol content of .19 percent two hours after the crash, and was also driving with a suspended license. On June 8, 2009, Gallo entered a not guilty plea at his arraignment. After a two-week jury trial held in September 2010, Gallo was convicted on three counts of second-degree murder, two counts of driving under the influence causing great bodily injury, and one felony count of hit-and-run. A bench trial found Gallo guilty of driving on a suspended license. On December 22, 2010, Gallo was sentenced to 51-years-to-life in jail.