Invited by the Phillies to spring training in 1964 as a nonroster player, Danny Cater was one of six rookies who made the team. He was the best pinch-hitter in the league among those who appeared more than 15 times, hitting .368 and finished the season with an overall .296 batting average.

In December, the Phillies got Dick Stuart from the Red Sox, and Cater was traded to the Chicago White Sox. In 1966 he was traded to the Kansas City Athletics and opened the 1967 season as the Athletics’ starting first baseman. In early May his .339 average garnered him The Sporting News’ American League "Player of the Week" honors. His season ended, however, when he was beaned by a fastball.

Cater's average climbed to .290 second only to Carl Yastrzemski’s league-leading .301 average in 1968. He made only five errors at first base for a .995 fielding percentage, the highest in the league, but George Scott won the "Gold Glove" Award for first basemen.

After the 1969 season, Cater was traded to the Yankees where he hit .301 in 1970, a career-high, and his 76 RBIs also matched his career high. But he began the 1971 season badly, with his average in the low .200s, but finished hitting .276

During spring training of 1972 Cater and infielder Mario Guerrero were traded to the Red Sox for pitcher Sparky Lyle.

Things didn’t begin well for Cater. With a batting average of .120 after 14 games, he was benched. His average was in the low .200s as the season went into July, and he received rough treatment from the Sox fans. Adding fuel to this perception was the fact that Sparky Lyle kept saving and winning games for the Yankees on his way to capturing the 1972 "Fireman of the Year" Award.

Although Cater’s hitting stroke returned briefly in July, he was benched after hitting .221 and was replaced at first base by Carl Yastrzemski. He appeared in only one game after August 20th and finished the season with a .237 average and 39 RBIs, both career lows.

Despite his being the subject of winter trade rumors, Cater was still on the Red Sox’ Opening Day roster for the 1973 season. He began to play more regularly and responded well at the plate with a career-high .313 BA.

In March 1975, he was traded to the Cardinals for minor leaguer Danny Godby. The bitter nature of Cater’s Boston experience was reflected in a newspaper column by Ray Fitzgerald in the Boston Globe after his departure: “Cater is gone from Boston. … Danny just drifted away. Unhonored and unsung. … He’s gone and not one voice was raised in protest."