He was the first professional athlete ever signed out of Crenshaw when the Montreal Expos selected him in the second round of the 1972 Major League Baseball draft.

Gary Carter, who was an All-Star and finished second in National League Rookie of the Year balloting as the right fielder for the Expos in 1975, was shifted back to his natural position behind the plate for the 1976 season to make room for Valentine in right field.

[9] Valentine was the first to emerge as a star; he was the Expos' sole representative at the 1977 Major League Baseball All-Star Game at Yankee Stadium.

Already recognized for his exceptional arm by this point in his career, he engaged in a throwing contest before the game with the Pittsburgh Pirates' Dave Parker, Los Angeles Dodgers' Reggie Smith and San Diego Padres' Dave Winfield, who were also recognized as having the finest throwing arms in the National League.

In the eighth inning, with two outs and a runner on second, Valentine drew a walk, bringing future Hall of Famer Tony Pérez to the plate.

[16] With slugging third baseman Larry Parrish also out of the line-up with a wrist injury, the Expos did surprisingly well in Valentine's absence; they went 21–16 to take a half-game lead in the National League East over the Phillies.

When Valentine finally returned on July 10, he began wearing part of a football face mask on his helmet.

[17] He batted .331 with nine home runs and 39 RBIs over the rest of the season, as the Expos found themselves in a three-way race with the Pirates and Phillies in the NL East.

[21] He was still on the disabled list when the Expos dealt him to the New York Mets for Jeff Reardon, Dan Norman and a player to be named later.

Almost as if National League pitchers realized they had nothing to fear from Valentine's bat, Kingman began seeing tougher pitching again, and hit just three more home runs over the rest of the season.

[24] As things turned out, the Mets narrowly avoided losing one hundred games, and finished last in their division.

[25] From there, Valentine put up respectable numbers, seven home runs and 33 RBIs, through the Mets' 6–4 victory over the Chicago Cubs on August 13.

Valentine was selected by just one team, the Pittsburgh Pirates, in the MLB re-entry draft when he became a free agent at the end of the season.

[35] He played well, batting .314 with ten home runs and 33 RBIs in 46 games for the triple A Oklahoma City 89ers, but it did not translate to major league success when he joined the Rangers in September.

[42] In September 1986, a year after retiring, he moved from Southern California, and immediately entered drug rehab in Phoenix, Arizona.

Valentine currently lives in the Dallas-Fort Worth area, where he works as a counselor at a local church and is a member of the Texas Rangers Alumni Association.

[43] He is the co-founder and president of PastPros, a web-based service that allows sports fans to purchase memorabilia directly from retired athletes.

The program also provides a lawn service for disabled seniors in the Dallas, Fort Worth area and surrounding communities.