After the war, Lex de Herder came into contact with press photographers via his job. This inspiring working environment made him decide to become a press photographer himself. He applied for a job at E.A. (Erik) Hof's Centraal Foto Persbureau (Central Photo Press Office), and on 11 May 1948 passed an exam at the Nederlandse Vereniging van Fotojournalisten (Dutch Society of Photo Journalists).
Early in 1949, Lex de Herder was sent to Indonesia as a conscript. He was stationed in Surabaja (Eastern Java) where from 15 February until 15 July 1950 he was a radio announcer on the programme for the Dutch troops at Radio Republik Indonesia Serikat and taught photography to the soldiers.
After his repatriation, he did not return to the Centraal Foto Persbureau, but started to work for Johan Klaver's photo office in Rotterdam. For sixteen years he was employed there as an advertising and corporate photographer, until he was asked to work for the daily newspaper Rotterdams Nieuwsblad in 1967. There he provided the photo's for Ton Schuurmans' weekly column 'Ter Plaatse' (On Location). On 1 September 1969, Lex de Herder was appointed photo editor of the newspaper and subsequently made fewer photo's himself.
In 1989 he went into early retirement. As he was Rotterdam's town photographer, the part of his archives documenting the city and its inhabitants was considered to belong in the Rotterdam Municipal Archives. The remaining archives were transferred to the nfa. These contain amongst others nature photographs and reports of his annual trips to Paris.