Tolan was a reserve outfielder during his years with the Cardinals, with whom he won a World Series title in 1967. He also played on the 1968 National League champions; however, the Redbirds lost to the Detroit Tigers in the World Series in seven games, after leading three games to one. The day after the final game of this Series Tolan was traded to the Cincinnati Reds along with reliever Wayne Granger for veteran outfielder Vada Pinson.
As their center fielder, often batting second behind Pete Rose and in front of Alex Johnson in the Reds lineup, Tolan in 1969 hit .305 and established career highs in home runs and runs batted in (21 and 93 respectively). In this, the first year both leagues were split into two divisions, the Reds finished third in the National League West, four games behind the division-winning Atlanta Braves. The "Big Red Machine", which also featured future Hall of Famers Johnny Bench and Tony Pérez (and would later feature a third, Joe Morgan), was just beginning to take shape.
In 1970, Tolan batted a career high .316 with 16 home runs and 80 RBIs, and led the National League in stolen bases with 57 (the only time former Cardinal teammate Lou Brock did not lead the National League in steals between 1966 and 1974) for a Reds team that won the National League West title for their first postseason berth since the 1961 World Series. The Reds swept the Pittsburgh Pirates in the NLCS in three games; in the second game, Tolan scored all three runs in a 3-1 victory, including hitting a fifth-inning home run off starter Luke Walker. However, the Baltimore Orioles defeated the Reds in the World Series in five games. Tolan went 4-for-19 in the Series, including a home run off Mike Cuellar in Game Two.
Tolan missed the 1971 season after rupturing his Achilles tendon playing basketball, which violated a specific clause in his contract barring him from that activity. He came back in 1972, winning both the Comeback Player of the Year award and the Hutch Award after batting .283 with 82 RBIs and 42 stolen bases. Tolan became only the second player to win both the Hutch Award and his league's Comeback Player of the Year Award (Tony Conigliaro was the first) and the first to do so during the same season. His Reds again defeated the Pirates in the NLCS (this time with the winning run scoring on a wild pitch by Bob Moose, after the Reds entered the ninth inning trailing by a run) to win the pennant; however, they were defeated by the Oakland Athletics in the World Series in seven games.
1973 was a disastrous year for Tolan, one in which his batting average plummeted to .206. He became a malcontent and had several squabbles with Reds management, who were still unhappy with his 1971 basketball injury. Tolan also went AWOL for two days in August and broke team rules by growing a beard. On September 27, the team suspended Tolan for the remainder of the season. The Reds won yet another division title but the suspension forced Tolan to miss the NLCS, which the Reds lost to the New York Mets. At the end of the season the Reds traded Tolan to the Padres for pitcher Clay Kirby. After the trade the Major League Baseball Players Association filed a grievance on Tolan's behalf. During the 1974 season, in which he batted .266 in 95 games, he learned that he had won his grievance. Tolan demanded that the Reds publicly apologize to him because his name had been slandered but never got the apology.
Tolan was released by the Padres after batting .255 in 1975. He signed with the Philadelphia Phillies as a free agent, and in 1976 batted .261 as a part-time outfielder. The Phillies won the National League East title to earn their first post-season appearance since the "Whiz Kids" were swept by the New York Yankees in the 1950 World Series. However, Tolan's former team, the Reds, defeated the Phillies in the NLCS.
Tolan played professionally in Japan in 1978. He was also a coach for the Padres from 1980–1983. During the strike of 1981, Tolan was dispatched to Walla Walla, Washington, where he was Tony Gwynn's first hitting coach.  Tolan also was player-manager of the St. Petersburg Pelicans, a team in the Senior Professional Baseball Association, in the two years of the league's operation, 1989–1990.
In his major league career, Tolan batted .265 with 86 home runs and 497 runs batted in, in 1,282 games played.
Tolan is married to Marian Trahan and they have a son Robert (Robbie) Tolan who plays professional baseball in the Washington Nationals organization. On December 31, 2008, Robbie was shot by a Bellaire, Texas policeman allegedly investigating reports of a stolen car after a confrontation in the Tolan driveway. The younger Tolan was unarmed and driving his own vehicle. The bullet lodged in Tolan's liver; the injury may have ended his professional baseball career. An investigation into the shooting is ongoing.
Tolan's cousin, Eddie Tolan, was a sprinter who won two Gold Medals in the 1932 Summer Olympics.