Massachusetts Governor Deval L. Patrick ’78 on Wednesday named William “Mo” Cowan, his former chief of staff and trusted aide, to serve as interim U.S. Senator for the next five months.

Cowan will occupy the Senate seat resigned Tuesday by newly confirmed Secretary of State John F. Kerry until the seat is filled in a special election on June 25.

Cowan, 43, was a partner at a Boston law firm and a former special prosecutor for the Middlesex County District Attorney’s office when he was recruited by Patrick to join the gubernatorial administration in 2009.

After spending two years as the governor’s chief legal counsel, Cowan became Patrick’s chief of staff in January 2011, serving for another two years before departing last month.

“Mo is a highly respected public citizen,” Patrick told reporters at a Wednesday press conference announcing the selection. “The people of the Commonwealth have benefitted from his wisdom and good judgment during his time in our office and will again in the Senate.”

The search for a replacement for Kerry, who had served in the Senate since 1985, began almost immediately after last November’s election. Kerry, who had travelled abroad for the Obama administration in a diplomatic capacity and had grown close to the president while playing the role of Mitt Romney in debate preparation, emerged as a contender for Secretary of State almost immediately.

He was nominated in December and approved by the same leading Republicans who killed the candidacy of Susan E. Rice, ambassador to the United Nations. Rice had been the frontrunner for the post until she was derailed by criticism of her comments about the September 2012 attack on the U.S. embassy in Bengazi, Libya.

Since then, the hunt for a replacement in Massachusetts intensified, and so did the competition.

The recently retired Barnett “Barney” Frank ’61-’62 publicly petitioned Patrick to consider him for the temporary seat, and the governor himself acknowledged that he received more than a few phone calls from interested parties. Some observers even called for Patrick to appoint himself.

Cowan did not emerge as a leading candidate in the public eye until late in the game. Frank succeeded in generating early media attention with his public appeal, and many believed that Victoria R. Kennedy, the wife of the late Senator Edward M. Kennedy ’54-’56, was a frontrunner.

Throughout the entire process, Patrick and his search committee remained tight-lipped.

The night before the appointment, media outlets named Cowan and Kennedy as likely frontrunners, but even those who dared to make a prediction quickly acknowledged that Patrick could pull a name out of left field.

It was not until just before Patrick’s scheduled press conference Wednesday that the selection was finally leaked in a report by the Boston Globe.

Cowan, who will travel to Washington, D.C., by early next week, said he does not intend to use his five months on Capitol Hill as a springboard for future political ambitions.

“This is going to be a very short political career,” he said. “I am not running for office, and I am not a candidate for public service anytime, today or in the future.”

—Staff writer Matthew Q. Clarida can be reached at clarida@college.harvard.edu.