He later played an instrumental role in OSU defeating Oscar Robertson's Cincinnati Bearcats team, scoring 12 points, with five rebounds.

In one game, while being scouted by coach Greene, he was called on to pitch in relief with a 1-0 lead in the bottom of the ninth inning, with the bases loaded and no outs.

[13][11] He made his Major League Baseball debut with the club that day against the Washington Senators, coming into the game in the bottom of the 8th inning as a defensive replacement for shortstop Chuck Oertel.

[15] He picked up his first big league hit (a single to left field) in a 3–2 Orioles win over the visiting New York Yankees on September 21.

[19] Adair's first full season in the big leagues came in 1961 with the Orioles, hitting .264 with 9 home runs and 37 RBI in 133 games.

[25][26] Aparicio and Adair ultimately played three years together (1963-1965), and are considered one of the top middle infield combinations in baseball history.

The resulting laceration required 11 stitches, but Adair was back in the park in uniform in time to play the entire second game.

As a result, Adair finished 17th in the AL MVP vote that was won by Zoilo Versalles of the Minnesota Twins.

[30] On June 13, 1966, Adair was traded by the Orioles with minor leaguer John Riddle to the Chicago White Sox for Eddie Fisher.

[16] With his pennant push performance at the plate, coupled with stellar defensive play, Adair finished 15th in the AL MVP balloting.

[36] In 1970, Adair hit just .148 (4-for-27) before being abruptly released on May 5 as he was about to board a plane to make a road trip to Baltimore with the team.

Adair had spent much of spring training that year with his six-year-old daughter, Tammy, who had terminal cancer and eventually died[37] and claimed the Royals let him go without taking the family's problems into consideration.

Royals General Manager Cedric Tallis defended the team's decision to release Adair saying "We just felt he couldn't help the club".

[38] In 1970, he also played 33 games for the Tulsa Oilers of the Triple-A American Association, in the St. Louis Cardinals minor league system.

In 1971, Adair finished his professional career playing 90 games for the Hankyu Braves in the Japan Pacific League.

[40] Adair later coached for the Oakland Athletics (1972–74, their World Championship years) and the California Angels (1975), working under his former Oriole teammate and manager in Boston Dick Williams in 1972-1973 and 1975, and for Alvin Dark in 1974.

His .985 lifetime fielding percentage at second base is better than his hall of fame contemporaries Nellie Fox, Joe Morgan and Bill Mazeroski.