Born in New York City, Foy was signed as an amateur free agent by the Minnesota Twins in 1962, then was later selected in that year's minor league draft by the Boston Red Sox.

Playing with the Toronto Maple Leafs in the International League in 1965, Foy was voted the Most Valuable Player and Rookie of the Year, and also won the league's batting title, hitting .302.

In his first year in the majors, with Boston in 1966, Foy batted a solid .262, drew the second-most walks in the American League (91), had a .364 on-base percentage, good for eighth in the junior circuit; he also scored 97 runs, fifth in the league. As pitching became more dominant in the late 1960s, Foy's numbers dropped.

In 1967, while receiving over 100 fewer at-bats, Foy batted a slightly worse .251/.325/.426 (his walk total halved); the league itself dropped by 4 batting points, 2 on-base points, and 18 slugging points. Foy set a career-high for home runs with 16.

In 1968, the "Year Of The Pitcher" (when Carl Yastrzemski led the league with a .301 batting average, and the American League batted just .230), Foy's raw statistics were (.225/.336/.326), his on-base percentage was 39 points above the league average, and his slugging and batting averages were roughly the same as the league average. He stole 26 bases that year and drew 84 walks.

The Red Sox left Foy unprotected in the 1968 expansion draft where the Kansas City Royals selected him with the fourth pick. He had a fine season in 1969. While the league still only batted .246/.321/.369, Foy's numbers were .262/.354/.370. He also had 71 runs batted in, his career-high.

Then, in a move considered by some to be one of the best trades in Royals history, Kansas City traded Foy to the third baseman-hungry Mets for Amos Otis and Bob Johnson. Otis developed into an All-Star, and an occasional MVP candidate. The Royals then traded Johnson the following year, after a 200 strikeout season, to the Pittsburgh Pirates for shortstop Fred Patek, who became another cornerstone of their rising franchise.

Foy posted a career-best .373 OBP while hitting .236/.373/.329 with 6 home runs and 37 RBI in 322 at-bats with New York.

His best day with the Mets, and perhaps of his entire career, came on July 19, 1970 when he went 5-for-5 with a double, two home runs, and five runs batted in as the Mets edged the Giants, 7–6, in 10 innings at San Francisco.

Although his averages were not that far off from his career average, Foy was considered a disappointment to Mets fans. Additionally, according to Mets pitcher Jerry Koosman, Foy fell under the influence of his old friends in the Bronx. In the first game of one doubleheader, Koosman and others thought he was high on some kind of drug (eventually confirmed to be marijuana), especially when he walked in front of manager Gil Hodges in the dugout during a pitch and started cheering. Still, Hodges started him at third base in the second game. The first batter hit a hard ground ball by Foy. He never even saw it, but even after it went by him, he kept punching his glove and yelling, "Hit it to me, hit it to me." Koosman and others all wanted Foy out right then, but according to Koosman, Hodges left Foy in the game just a little longer to show that he didn't fit on the team.^{[citation needed]}

After the season, the Mets left him off the roster, and the Washington Senators drafted him in the Rule 5 draft.^[1]

Foy was only an average batter with the Senators batting .234/.363/.297 in 128 at-bats. He was sent to the minors in May. After batting .191 in 15 games,^[1] he was released on July 16, 1971 and never played in another professional baseball game.^{[citation needed]}

Towards the end of his career his performance was hindered by substance abuse, which continued after his retirement.^{[citation needed]}

Foy was later a counselor for troubled youth in his native New York-area.^{[citation needed]}

He died of a heart attack at his home in New York on October 12, 1989, at the age of 46, and was buried in Woodlawn Cemetery in the Bronx.^{[citation needed]}

• Career statistics and player information from MLB, or ESPN, or Baseball Reference, or Baseball Reference (Minors)