Mullen worked as a house carpenter in Bellingham, WA, in 1910, but had switched to farming by 1920. Principal, S.S. Mullen, Sr., Building Contractor, Port Angeles, WA, c. 1926;

Although U.S. Census records on his nationality differ, (some say he came from England, others English-speaking Canada), Stewart S. Mullen, Sr., was born c. 1889 probably in Nova Scotia; According to the U.S. Census of 1910, Mullen lived with his parents in a dwelling at 2626 Patton Street in Bellingham, WA. The 1920 U.S. Census listed Mullen as a farmer living with his wife, Theo B., on acreage off of Smith Road in Ten Mile Township, Whatcom County, WA. In 1930, Mullen lived with his wife and two children at 529 East 2nd Street in Port Angeles, WA;

Records on his parents nationality conflict. According to U.S. Census data, his parents came from either England or English-speaking Canada, probably Nova Scotia. His father, E.J., (born c. 1846) worked in a Bellingham, WA, lumber mill as a laborer in 1910; his mother, named either E.J. or C.J., (born c. 1850 in either England or Nova Scotia) worked in the home. They married c. 1873.

His wife was Theo (probably short for "Theodora") B. Mullen (born c. 1891 in WA); they were married c. 1914, she at age 22, he at 24. Her father may have been English-Canadian or from ME, her mother was from IL.

In 1930, two children, a daughter, Wilder, (born c. 1921 in WA) and son, Stewart S., Jr., (born c. 1923 in WA).