On his father's side he was the grandson of Andreas Zaimis, another former prime minister of Greece, and related to the great Kalavrytan family with notable participation in the Greek War of Independence from 1821.

Alexandros became involved in politics after the death of his father who was the elected member of parliament for Kalavryta.

He was appointed prime minister under King Constantine I to succeed Venizelos in October 1915, but resigned a month later when his government failed to receive a vote of confidence.

In 1917, Zaimis served again as prime minister under King Constantine I, while Eleftherios Venizelos led a rival government controlling northern Greece.

However, only one year into his second term, he was thrown out of office by Georgios Kondylis, who abolished the Republic and proclaimed himself regent pending the results of a referendum on restoring the monarchy.