Mickelson has won two straight tournaments -- by 13 shots at the BellSouth Classic and two shots at Augusta -- as well as two consecutive majors. He was last ranked second in the Official World Golf Rankings at the beginning of 2003.
Tiger Woods is the No. 1 player in the world for the 386th time, and is followed by Mickelson, Vijay Singh, Retief Goosen and Ernie Els. Mickelson's jump pushed Singh and Goosen down one spot apiece.
Sergio Garcia remained sixth, trailed by David Toms, Jim Furyk, Luke Donald and Adam Scott. Toms replaced Furyk at No. 7 even though he missed the cut at Augusta, while Furyk played into the weekend.
Chris DiMarco also missed the Masters cut and dropped out of the top 10, falling one slot to 11th. Jose Maria Olazabal tied for third on Sunday and moved up four places to No. 12, where he was followed by Henrik Stenson, Chad Campbell and Tim Clark to round out the top 15.
Clark finished alone in second at the Masters, and his jump to No. 15 came after he was ranked 30th last week. Campbell joined Olazabal in third place and moved up six places.
Rounding out the top 20 were David Howell, Darren Clarke, Colin Montgomerie, Geoff Ogilvy and Rory Sabbatini.