Born in Berlin on 15 December 1896, Otto Nothmann was the fifth and youngest child of coke factory manager Adolph Nothmann (1860-1913) and his wife, the former Alwine Lustig, who had settled in the city in 1889. After his father's early death, Nothmann commenced studies in the Faculty of Architecture at the Technische-Hochschule (Technical University), qualifying as Diplom-Ing Architekt (graduate engineer-architect) in November 1922. The following year, he married Elizabeth Ilse Kathe Silber; the couple would have one daughter, Eva Margarethe Nothmann, born in 1925. Between 1926 and 1929, Nothmann worked in the offices of several architects in both Berlin and Hamburg. In 1929, he became registered to practice under his own name in Berlin and, for the next nine years, undertook works that included (by his own account) 'erecting and rebuilding of office and residential buildings, country houses, interior decoration and furniture'. Much later, one of Nothmann's Berlin houses - a smart two-storey rendered villa with hipped pantiled roof - would be published in the Australian Home Beautiful, which, due to the wartime political situation, was diplomatically described only as a house 'erected in a suburb of one of the European capital cities'. Nothmann himself resided for some years at Claudiusstrasse 11, between the River Spree and the Tiergarten, before moving further outwards to Wilmersdorf on the city's south-west fringe.
Nothmann's flourishing practice in Berlin came to an abrupt end in November 1933, when the National Socialist regime passed legislation that prevented Jewish architects from being able to practice in Germany. Declared as volljude (literally, 'full Jew'), Nothmann's application to become a member of the Reich Chamber of Fine Arts & Culture - necessary to continue practice - was rejected. He subsequently fled the country with his wife and teenage daughter. The family then spent several years in London, residing at 5 Erskine Hill, Hampstead, before leaving for Australia (via India and the Dutch Indies) on 10 March 1939 aboard the MS Sibajake. They evidently changed ships en route, as it was another vessel, the Marella, that brought the Nothmanns to Sydney on 24 April 1939.
The family soon relocated to Melbourne where, in May, Nothmann was offered a position in the office of Stephenson & Turner. He was to remain there for almost a year, until April 1940. According to a written testimonial from senior staff member Ellison Harvie, 'Mr Notman [sic] was employed on general detail drafting and during his employment in this office he largely overcame the language difficulty and acquired a good working knowledge of local building construction'. From 1941 to 1944, he was employed in the Physics Department of the University of Melbourne, where he worked under Professor Laby as an instrument draftsman 'for various war equipment and teaching equipment'.
In October 1944, Nothmann transferred to the Architects' Branch of the Public Works Department (PWD), where he worked on a broad range of projects including tuberculosis hospitals, schools, police stations and residential work. His involvement with hospital projects - which would have been influenced by his period in the offices of Stephenson & Turner - included developmental work on the design of hospital equipment and improved ward layouts. During this time, Nothmann evidently also maintained his own practice, working on 'various private works of residential, commercial and industrial type'. No specific examples, however, have yet been identified. In May 1949, Nothmann left the PWD to take up a position as an architectural assistant in the office of Bates, Smart & McCutcheon. By the mid-1950s, he was evidently employed as a draftsman in the office of a fellow German-speaking Jewish emigre, Dr Ernest Fooks, as Nothmann's initials appear on the working drawings of a holiday house at Mount Eliza that Fooks designed in 1954.
Virtually nothing else is known of Nothmann's professional career, and only scant details of his personal life. In 1941, he sponsored the emigration of a relative, Hans Michael Nothmann, from Germany. Nothmann himself became a naturised citizen in May 1944, after living five years in Australia. His wife, who anglicised her name as Kate Elizabeth Nothmann, was a psychologist who also worked as a handwriting analyst in court cases. For many years, the couple lived in South Yarra. Nothmann died on 4 September 1967, survived by his wife and daughter. Nothmann's daughter, Mrs Eva Urbach, has published a memoir that covers her family's flight from Nazi Germany, entitled Through Coloured Glass: Reflections on My Changing World (2008).