Lennon was born at Marysville, California, on February 25, 1866, and educated in the public schools.

During high school, he studied at the Sacramento Institute, where at commencement in June 1881 he presented a speech on "right of free discussion.

[7] He moved to San Rafael, California, in Mill Valley, and in 1902 was elected judge of the Marin County Superior Court on the Republican ticket, filling the vacancy left by Frank M. Angellotti's elevation to the California Supreme Court.

[13][14] During the campaign, he presented a proposal to streamline court cases, and was endorsed by Republican Senator Hiram Johnson.

[15][16] In 1919, he was elected over incumbent William G. Lorigan as an associate justice of the Supreme Court, along with Curtis D. Wilbur who won re-election.