During the years 1930 to 1933 he travelled widely, particularly in Africa, in search of big-game (as the trophies that could be seen at his house at Princes Gate after the war testified) though his bags were reputedly always packed with art books.

Unusually, perhaps, his subsidiary subject at university was Kinderpsychologie (Child Psychology), taught by a lady who was a pupil of Sigmund Freud and who was vouched for by his friend Jan van Gelder.

At the annexation of Austria by Nazi Germany in 1938, and because of his British citizenship (he had apparently hung the Union Jack from his house in Vienna),[3] Seilern decided in 1939 to return to England, bringing with him his already large art collection and his library.

Based in England now he was able to provide finance to support to another art historian fleeing from Nazi-occupied Austria, Ludwig Münz, as well as helping Johannes Wilde and his Jewish wife to leave Vienna.

Seilern then enlisted in the ranks of the British Army (though aged thirty-eight), serving in the Royal Artillery, and in 1940 immediately volunteered for the disastrous Russo-Finnish campaign, only escaping from occupied Norway.

Seilern returned to London at the end of the war to live in a great gloomy house he had acquired at 56 Princes Gate, South Kensington, where he rarely opened the blinds and electric light was largely eschewed as he said it distorted colours in pictures.

The war over, he resumed collecting works of art, buying them privately for the most part rather than at auctions, never from a photograph, and never when pressed by a dealer, pictures left 'on approval' frequently being returned.

He devoted time to studying them in depth and cataloguing them accurately, though he was also a generous anonymous benefactor of public collections; for example, he lent to exhibitions held at the British Museum (he lent his Michelangelo drawings to the 1975 exhibition there – which he would otherwise not have done except that it was being held in honour of his friend Johannes Wilde), and in 1945 he gave the National Gallery anonymously a very fine full-length portrait of William Feilding, 1st Earl of Denbigh by Sir Anthony van Dyck [NG5633] as well presenting anonymously to the British Museum in 1946 the majority of the important collection of Old Master drawings (some 1250 in number) belonging to Mr. Thomas FitzRoy Fenwick, which had been assembled by Sir Thomas Phillipps, 1st Baronet in the library at Thirlestaine House, Cheltenham, and which had been catalogued by Arthur E. Popham of the museum in 1935[4] and bought by Seilern en bloc (Seilern retained about two dozen drawings).

Eventually his collection would come to include masterpieces not just by Rubens (32 paintings – oil sketches and modelli mainly, but also some of the artist's copies of Old Masters like Raphael's portrait of Baldassare Castiglione – and 20 drawings); 14 small copies by David Teniers the Younger after pictures in the collection of Archduke Leopold Wilhelm of Austria during his Governorship of the Spanish Netherlands in 1647–56, commissioned to make a series of prints for the book known as the Theatrum Pictorium first published in 1660; and 12 by Tiepolo (including six oil sketches for paintings for the Monastery Church of Aranjuez, south of Madrid), and pictures by Claude Lorrain, Degas, Van Dyck, Quentin Massys, Lorenzo Lotto, Magnasco, Manet, Francesco de Mura, Palma Vecchio, Parmigianino, Pittoni, Sebastiano Ricci, and Tintoretto.

Surprisingly perhaps, he commissioned a ceiling painting in triptych form of The Myth of Prometheus for the entrance hall of his home in Princes Gate by his friend Oskar Kokoschka.

He is recorded as owning a three-quarter length pastel of his mother by Clemens von Pausinger [de] (1894) as well as a signed and dated bust-length portrait of himself as a boy by Muller-Ury, which was unframed and stored in a cupboard.

This included his Michelangelo drawings, his Rubens collection, his Tiepolos, Parmigianino's Virgin and Child and Rest on the Flight into Egypt, the Kokoschka ceiling and Bernardo Daddi's masterpiece Virgin and Child with Saints from 1338, which Seilern had purchased in 1956, as well as modern works by Pissarro, Edgar Degas, Pierre-Auguste Renoir and others, and two pictures by Daubigny and Narcisse Virgilio Díaz he had retained from his grandmother's collection in New York.

Seilern never married, was extremely reserved, and his private life is undocumented although he had many women friends, and was evidently fond of his brother Charles's four children, his cousin Paul Methuen (whose pictures he bought occasionally, apparently never keeping them but giving them away as presents), and enjoyed long friendships with scholars like Wilde, Ludwig Burchard, Anthony Blunt, Michael Kitson, and unusually, as he was perforce also a dealer, James Byam Shaw.

He was a large man with quite a booming voice, and according to James Byam Shaw possessed something of a dual character, being business-like and intellectual with his male friends, but with women of all ages had all the charm and manners of an aristocrat from the former Austro-Hungarian Empire.