。
Mother once won a swimming championship.
Literal
Mother [topic-は] once swimming [in-で] won [experience-ことがある].
~たことがある is the dedicated 'have done X (at some point in the past)' experience construction — distinct from a simple past, which asserts a specific event. The adverb かつて ('once, formerly') is a written/formal flavor — もう昔 or 昔 would feel more conversational. で marks the domain/field of the winning, not an instrument.