Why is it der Washingtoner?

Washingtoner 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

Coming from Washington, belonging to Washington

[1] aus Washington kommend, zu Washington gehörend

Examples

[1] We went to the 87 steps to the Lincoln Memorial, the most famous Washington Sightness

[1] Wir gingen die 87 Stufen zum Lincoln Memorial, der bekanntesten Washingtoner Sehenswürdigkeit, hinauf.

Declension

Positiv Komparativ Superlativ
Washingtoner — —

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary