Despite what you might hear Mark Twain never said that the coldest winter he's ever spent was a summer in San Francisco. To the contrary, he called San Francisco Paradise and reserved his weather critique for Paris.

Twain arrived here in 1864 and stayed for about two years. It's true, summer temperatures during that period went as low as 57F (13.4C), which is pretty cold.
Perhaps that prompted him to stay inside with Bret Harte, Ambrose Bierce, Henry George, Ina Coolbrith, and Joaquin Miller. In the warm and cozy Turkish bath on Montgomery Mark Twain met a man named Tom Sawyer and often played cards with him. In later years this man advertised his tavern as belonging to "the original Tom Sawyer".
It is certainly here, under Bret Harte's tutelage, that he finished his transformation from Samuel Clemens to Mark Twain. Harte even got Twain his job on the Californian, where he was editor. Harte said of Twain: “I think I recognize a new star rising in this western horizon.” Later this friendship fell apart. Twain said bitter and cruel things of Harte. Harte said nothing about Twain.

But there were many friendships. San Francisco at the time was abloom with poets, actors, and open-handed mecenates. Mark Twain considered it "the most cordial and sociable city in the Union", and wrote to his mother that he felt as much at home here as in his home town of Hannibal.

At first Twain tried his hand at mining speculation. During that period he lived in fancy hotels on Montgomery St (it seems to have been the street where he spent most of his time in SF) and explored more than he wrote. But once his stocks failed he had to go back to writing.
He took a job as the only reporter of the Morning Call raking the town for news and visiting as many as six theaters per evening to report on the plays. There was not a story in town that Twain did not milk. He even tried to ascribe a new relationship to the recently bereaved dog Bummer, writing that he was seen with a “vagrant black puppy”.
During this period he wrote only one article with true feeling, a story about some Irish bullies beating a Chinese man. That story was killed by the editor in deference to the taste of the Call's public. Soon the Call's editor fired Twain, hurting his feelings but freeing him to write on subjects more expressive of his talent for half a dozen other periodicals.

He did write often about San Francisco weather, particularly on the effect of wind on the feminine fashions on the time. If you are lucky enough to be here on a windy day, stand on the corner of Clay and Montgomery and imagine hoop skirts flying up like glorious parachutes of lace. Mark Twain will be with you in spirit.

In December 1864 Twain's constant criticism of the local police proved so well-founded that he had to leave town for two months for his safety, and went to Calaveras County, known, of course, for its jumping frogs. Twain's report on those was his first work to be published in a national publication and jump-started his fame. A year later he delivered his first public lecture at Maguire’s Academy of Music and left San Francisco in 1866 with two of his careers, writing and lecturing, in full swing.

In 1868 Mark Twain returned briefly to write Innocents Abroad and left again, never to return.