Ancien = former/old (French Adjectives that change meaning according to position)

Certain adjectives change their meanings depending if they appear before or after the noun.

The meaning of adjectives that can be placed before or after a noun in French

Here's the case of ancien:

L'ancienne maison où j'habitais était plus grande que mon appartement actuel.The (former) house where I used to live was bigger than my current apartment.

La maison ancienne où j'habite a beaucoup de cachet.The old house where I live has a lot of character.

Mon ancien petit copainMy ex-boyfriend

Ce tableau ancienThis ancient painting

Before the noun, ancien means former / ex- / no longer

After the noun it means ancient / old
The adjective ancien in the sense of "ancient" is not really used in French for people.

 

Remember that generally adjectives appear after the noun, but some very common adjectives go before (See Adjectives usually go AFTER nouns in French (Position of Adjectives) and Position of French Adjectives - Short and common adjectives that go BEFORE nouns)

See also: 

Propre = own/clean (French Adjectives that change meaning according to position) 

Cher= dear/expensive (French Adjectives that change meaning according to position) 

Certain = specific / sure in French (adjectives that change meaning according to position) 

Dernier = final/previous (French Adjectives that change meaning according to position)

Vrai = real/true (French Adjectives that change meaning according to position)  

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Learn more about these related French grammar topics

Examples and resources

Un ancien collègueA former colleague
Mon ancien petit copainMy ex-boyfriend
Ce tableau ancienThis ancient painting
Un vase ancienAn ancient vase
L'ancienne maison où j'habitais était plus grande que mon appartement actuel.The (former) house where I used to live was bigger than my current apartment.
La maison ancienne où j'habite a beaucoup de cachet.The old house where I live has a lot of character.
I'll be right with you...