A prominent figure in his hometown of Uddevalla, Malmgren became a colorful and well-known part of the city's history through, among other things, his long-lasting ownership of the newspaper Bohusläningen (The Bahusian), work in the local political scene, eccentric and extravagant lifestyle, and faux-medieval Tureborg Castle.

His father had taken control of the newspaper – previously named Uddewalla Weckoblad, and founded by the publisher Anders Johansson in 1826 – in 1843, bringing the publication into a new era of liberalism after a period of held-back conservatism.

The places he visited ranged from the Kingdom of Italy and the German Confederation (where his steamboat journeys on the Rhine would provide a significant inspiration in his later life), to the Australian colonies of the British Empire and the Pacific Islands.

Other than editorials and news items, Malmgren wrote and published the feuilleton Under södra hemisferens himmel (English: "Under the sky of the southern hemisphere"), later printed in book form, a romanticized tale of his youthful adventures in the Pacific archipelagos and deserts of Australia, where he reportedly had panned gold.

Bohusläningen soon became hugely successful, popular especially among the region's growing liberal and progressive milieu due to Malmgren's firm political editorial guidelines.

A conservative competitor appeared in 1887, in the form of Bohusläns Allehanda, but it didn't survive past 1891, leaving the region's "preservers of society" unable to make themselves heard against the "dissolving tendency" that, according to them, was the hallmark of the "terribly red" Malmgren.

At first intended only as a Summer home, it was extravagantly decorated, and after an expansion in 1901–1902 interiors designed by the artist Vicke Andrén (famous for, among other pieces, ceiling paintings in the Royal Swedish Opera) were among the additions.

The mock castle, inspired by his German voyages in his youth and located atop a hill overlooking Villa Elfkullen, was finished in 1912.

Malmgren held many grand feasts inspired by the national romanticism of the time in the castle, featuring for example fully roasted pigs and mead served in wooden tankards.

[1][5] Suffering from strong herpetophobia, Malmgren saw himself forced to clear the site of his castle from snakes, paying 1 krona and 25 öre – the cost of a litre of brännvin at the time – to every local who could produce a dead reptile.

[1] Other measures he supported was the improvement of social care, increased compulsory education, shorter working time, and the introduction of state pensions for workers.

After gradually retiring from the day-to-day business of his increasingly successful newspaper, he spent several years as Chairman of the Uddevalla City Assembly.