San Francisco Mayor Ed Lee will appoint North Beach neighborhood activist and businesswoman Julie Christensen to the Board of Supervisors on Thursday, filling a key swing seat on the 11-member board with a rookie officeholder and likely loyal ally.

The move is both pragmatic and risky for Lee, who spurned some of his longtime Chinatown supporters who preferred another candidate. It is also likely to draw the mayor’s nemesis, former board President Aaron Peskin, out of political retirement to take on Christensen in November, when Lee is on the ballot for re-election.

“I wanted to choose the best (person) that would represent not only Chinatown, but also all the other districts,” Lee said after a news conference in North Beach announcing Christensen as his pick. “I’ve done so with Julie.”

Lee, the city’s first Chinese American mayor, said he knew his decision not to pick a Chinese appointee to represent a diverse and dense corner of the city that includes Chinatown “was a consideration I had to weigh.”

“I do think that I made the best decision based on both experience, passion and their ability to carry out all of the work for the entire district,” Lee said.

Christensen, who has known the mayor for years, seemingly represents a reliable vote in Lee’s corner, at least until November, as he seeks to stave off any viable challengers while San Francisco wrestles with an acute housing crunch and other side effects of a tech-fueled economic boom.

Politically moderate Supervisor Scott Wiener called it “an inspired choice.”

“Her motivation is neighborhood improvement. Period,” Wiener said.

'Kind of awe-inspiring’

“When a person puts this kind of trust in you, it’s kind of awe-inspiring,” Christensen told media, neighbors and city brass assembled in front of the new North Beach library, a construction effort Christensen pushed for years as a neighborhood activist.

The setting was carefully chosen. Not only did it showcase one of Christensen’s most tangible public accomplishments, but it drew a sharp contrast with her expected challenger, Peskin, whose wife and Telegraph Hill allies fought the demolition and replacement of the old 1950s-era library for years.

Christensen, a Telegraph Hill resident and member of the advisory board for the smart-growth think tank SPUR, played a central role in getting the new North Beach library built and has championed the renovation of the adjacent Joe DiMaggio playground.

She is pushing to have the Central Subway, now under construction, extended to North Beach and Fisherman’s Wharf. Christensen also founded Surface Work, a product design consultancy that she will now close to focus on being a supervisor, which has an annual salary of $110,858.

Potentially a key vote

She is expected to be sworn in minutes after noon on Thursday, when the Board of Supervisors is scheduled to convene its first regular meeting of the year. That will allow Christensen to cast a potentially key vote for the next board president and, through a quirk in city law, also be able to serve 10 years in office if she can defend her seat against challengers like Peskin.

Planning Commission President Cindy Wu had been considered by many to be the front-runner to fill the seat representing San Francisco’s District Three, which includes the northeastern waterfront, major businesses in Union Square and the Financial District, the deeply layered ethnic enclave of Chinatown, and storied neighborhoods like North Beach, Telegraph Hill, Nob Hill and part of Russian Hill.

The seat has been vacant since former board President David Chiu was sworn in to the state Assembly in December. Chiu was elected in November to replace termed-out Assemblyman Tom Ammiano to represent the 17th Assembly District covering San Francisco’s eastern side.

A liberal planner

Wu is a community planning manager at the Chinatown Community Development Center, an influential affordable housing nonprofit with deep ties to Lee, a former tenants rights attorney in the 1970s and 1980s.

But some of the mayor’s advisers and major donors view Wu as considerably more liberal than the business-friendly Lee, who since becoming mayor in 2011 has embraced the tech industry and made job growth — and then housing construction — his core priorities.

While on the Planning Commission, Wu in 2012 voted against a luxury condominium development diagonally across the Embarcadero from the Ferry Building for which Lee was the public face.

The proposed development, known as 8 Washington, became an oppositional rallying point for Peskin and others on the city’s progressive left to fight tall development along the waterfront. That included the Golden State Warriors’ now-scuttled plan to build an arena on Piers 30-32, which Lee had called “my legacy project.” The NBA franchise decided in April to build its arena just off the waterfront in Mission Bay.

John Coté is a San Francisco Chronicle staff writer. E-mail: jcote@sfchronicle.com Twitter: @johnwcote