[2][3] Jefferies attended Junípero Serra High School in San Mateo, California, where he played baseball and football.

[3] He hit .367 with 20 home runs, 48 doubles and 101 RBI for Jackson in 1987, earning Jefferies a brief call-up from the New York Mets at the end of the 1987 season.

After spending most of the 1988 season at AAA (where he hit .282), Jefferies was recalled at the end of August and allowed to play out the year as a starter, mostly at third base.

The team had an MLB-best 24–7 record after his debut and finished with a league-best 100 wins en route to the National League East title.

In response to criticism from teammates, on May 24, 1991, Jefferies pleaded his case in an open letter read on WFAN, New York's sports radio station.

[3] In 1990, Jefferies raised his batting average to .283 while scoring 96 runs and leading the NL with 40 doubles, but the Mets finished 2nd for the second straight year.

That off-season the team traded him, along with McReynolds and infielder Keith Miller, to the Kansas City Royals for former All-Star pitcher Bret Saberhagen and utility man Bill Pecota, ending his stay with the Mets.

[6] He chose to leave St. Louis because the Cardinals would not give him a no-trade clause and he moved to the outfield for the Phillies, where he performed adequately over the 1995, 1996 and 1997 seasons, but injuries to his thumb and hamstring hampered his effectiveness.

On August 25, 1995, in a game vs. the Los Angeles Dodgers at Veterans Stadium, Jefferies became the first Phillie to hit for the cycle since Johnny Callison in 1963.