Why is it der Londoner?

Londoner ends in -er, and most nouns ending in -er are masculine: in a German dictionary, 86% of them take der.

All nouns ending in -er
Sense 1
der

Meaning

[1] A resident, resident of the city of London, a person born in London

[1] ein Einwohner, Bewohner der Stadt London, eine in London geborene Person

Examples

[1] The Londoners are hospitable people

[1] Die Londoner sind gastfreundliche Menschen.

[1] I am born in London

[1] Ich bin gebürtiger Londoner.

Declension

1 Singular Plural
Nominativ der Londoner die Londoner
Genitiv des Londoners der Londoner
Dativ dem Londoner den Londonern
Akkusativ den Londoner die Londoner

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary