In the 1980s, Greifeld was a district manager for
Unisys in
Jericho, NY. In 1991 he joined Automated Securities Clearance, Inc. (ACS), becoming president of the company in 1993. While at ACS he spearheaded the creation of BRASS, one of the first electronic stock
order matching systems, and Brut, an
electronic communication network (ECN). After
SunGard Data Systems acquired ACS in 1999, Greifeld was promoted to group CEO of SunGard Brokerage Systems. Under Greifeld, in 2007 Nasdaq attempted a hostile takeover of the
London Stock Exchange, but their offer was ultimately rejected by shareholders. In 2011, Nasdaq OMX collaborated with
IntercontinentalExchange to attempt a hostile takeover of
NYSE Euronext, however their bid was withdrawn due to antitrust concerns. In 2016, Nasdaq announced that Greifeld would be succeeded as CEO by
Adena Friedman as of January 2017. He continued in the role of chairman until May 2017, at which time he became the chairman of Virtu Financial. In 2019, Greifeld authored the book "Market Mover: Lessons from a Decade of Change at Nasdaq", chronicling his time at Nasdaq. Greifeld left Virtu in April 2023 to work on launching a tech-focused investment fund, Cornerstone FTM, among other endeavors. ==Personal life==