In 1924 Mörner comes in contact with the French poet André Breton´s surrealist manifesto (Mörner later becomes the theoretical and literary interpreter for surrealism among the Halmstad Group). From 1925 to 1928 he studies at Carl Wilhelmsson´s painting school in Stockholm and in 1928 spends a great deal of time in Halmstad where his father is the governor of the province of Halland. In conjunction with a project for the decoration of the castle in Halmstad he gets to know Esaias Thorén. Mörner forms the Halmstad Group at the end of August 1929 together with Axel Olson, Erik Olson, Waldemar Lorentzon, Sven Jonson and Esaias Thorén.
In 1930 Mörner takes part in Art Conret, a scandalous exhibition in Stockholm organised by Otto G Carlsund. Same year he paints Romantisk landskap (Romantic Landscape) with obvious surrealistic attributes. But it is first a few years into the ´30 that his work orients towards surrealism Stellan Mörner, after some years tending towards post cubism. Mörner exhibits, with the Halmstad Group, at Kubisme=Surrealisme in Copenhagen in 1935, a surrealist exhibition in London 1936 and the international surrealist exhibition in Paris 1938. Mörner´s surrealistic works often contain castle interiors and dramatic shadows — memories from childhood at Esplunda. Towards the end of the 30´s he paints a series of scenic dreamscapes, among others Drömland med hjärtan (Dreamland with Hearts) 1939.
Mörner paints a different kind of surrealism from 1943 to 1945. The images are retrospective, romantic, mixing time and space in a surrealistic manner; for example Ur min värld (From My World) 1944. People, instead of inanimate objects, are now the focus. These works may be viewed as an escape from the dark reality of WW II.
From 1944 onward, Mörner spends his summers in Söndrum. His home, Furet, is a gathering point for Söndrumskolonin (the Söndrum Colony) — a grouping of artists in Halmstad´s coastal area — along with Sven X:et Erixson, Felix Hatz and members of the Halmstad group. In the beginning of the 1950´s Mörner paints a number of geometric, colourful cliff landscapes.
In 1945 he is commissioned by Alf Sjöberg to make the costumes and décor for Shakespeare´s Twelfth Night at the Royal Dramatic Theatre, in Stockholm. This is the start of a long association with the world of theatre — Stellan Mörner makes over 30 set designs and costume designs between 1945 and 1967. In 1948 Mörner publishes the book Timmarna innan (The Hours Before). Other publications include Resa med flaggstång (Voyage with Flagpole) 1956, Spegel mot mitt liv (Mirror on my Life) 1969, and Det varma kvällsljuset (The Warm Evening Light) 1976. Mörner´s literary works also contain strong surrealistic allusions.
The end of the 1950´s and during the 60´s is an experimental period, with collage and spontaneous painting mixed with word fragments, often with a surrealist tone. In 1960 Mörner paints Hej på er allesammans! (Hello - All of You!), with his palm prints in various colours. Towards the end of the ´60s Mörner renews his ties to surrealism. In 1979 Mörner passes away; thereby dissolving the Halmstad Group.