Campaign aides said Solis Doyle made the decision to leave on her own and was not urged to do so by the former first lady or any other senior member of the team. But it comes as Clinton struggles to catch Obama in fundraising and momentum and faces the prospect of losing every voting contest yet to come in February.
Solis Doyle announced the shift in an e-mail to the staff on Sunday.
"I have been proud to manage this campaign and prouder still to call Hillary my friend for more than 16 years," Solis Doyle wrote. "Maggie is a remarkable person and I am confident that she will do a fabulous job."
Solis Doyle said she will serve as a senior adviser to Clinton and the campaign, and travel with Clinton from time to time.
Williams, who served as Clinton's White House chief of staff, is a longtime Clinton confidante who joined the campaign after the New York senator narrowly won the New Hampshire primary Jan. 8. She will begin assuming the duties of campaign manager this week.
The staff shake-up caps a week in which Clinton grabbed the bigger prizes on Super Tuesday, winning New York, California and New Jersey, but Obama prevailed in more contests. Obama won the popular vote in 13 states, while Clinton won in eight states and American Samoa.
Both Clinton and Obama raised a stunning $100 million each last year, but Clinton recently has lagged behind Obama in money collected. He raised $32 million in January to her $13.5 million, forcing her to lend her campaign $5 million before Super Tuesday. The campaign said Saturday that it had raised $10 million since the beginning of February.
Obama enjoyed a three-state sweep Saturday night, winning the Louisiana primary and caucuses in Washington state and Nebraska. He has the potential to pad his victories in contests Tuesday in Maryland, Virginia and the District of Columbia, as well as next week in Wisconsin and his native Hawaii.
Clinton is hoping to stem the tide on March 4 when Ohio, Texas, Rhode Island and Vermont vote.
In a statement, Clinton praised Solis Doyle and said she looked forward to her continued advice in the coming months.
"Patti Solis Doyle has done an extraordinary job in getting us to this point - within reach of the nomination - and I am enormously grateful for her friendship and her outstanding work," Clinton said. "And, as Patti has said, this already has been the longest presidential campaign in history and one that has required enormous sacrifices of everyone and our families.
"I look forward to her continued advice in the months ahead," Clinton added.
Williams joined the campaign when Clinton brought aboard a new group of advisers.
"Patti and I have worked with Maggie Williams for more than a decade," Clinton said in the statement. "I am lucky to have Maggie on board and I know she will lead our campaign with great skill towards the nomination."
The Democratic Party's system of awarding pledged delegates proportionally and the oversized role of superdelegates, the 796 lawmakers, governors and party officials who are not bound by state votes, meant that no candidate had a commanding lead.
According to The Associated Press' latest survey, Clinton had 243 superdelegates and Obama had 156. That edge was responsible for Clinton's overall edge in the pursuit of delegates to secure the party's nomination for president. According to the AP's latest tally, Clinton has 1,125 total delegates and Obama has 1,087. A candidate must get 2,205 delegates to capture the nomination.
The delegate numbers increased the possibility of a protracted fight for the Democratic nomination, perhaps lasting through this summer's national convention in Denver.