Parnell enjoyed his best season in 1949, going 25–7, leading the league in wins, complete games (27), and innings pitched (295+1⁄3), and finishing second with a 2.77 ERA.

It would be the final highlight of Parnell's career, which ended prematurely after the 1956 season due to a torn muscle in his pitching arm.

Parnell still holds the Red Sox career record for left-handed pitchers in games started, innings pitched, and victories.

Parnell once remarked that a southpaw's biggest challenge at Fenway Park was the limited foul territory, not the wall.

After his playing career, Parnell managed the New Orleans Pelicans of the Class AA Southern Association in 1959 and several Red Sox farm teams from 1961 to 1963.

He called the final out of the last regular-season game of the 1967 Red Sox "Impossible Dream" season on WHDH-TV:[2] "Little soft pop-up...Petrocelli will take it...he does!