。
She works as an assistant to the manager.
Literal
She [topic-は] manager of assistant [as-として] is-working.
~として ('as / in the capacity of') is the standard particle for assigning a role or function — 助手として働く ('work as an assistant'), 教師として ('as a teacher'), プレゼントとして ('as a present'). 支配人 ('manager') is the formal term for a head of an establishment, especially in hospitality (hotels, restaurants); in modern offices the loanword マネージャー is more common. 助手 ('assistant') is a Sino-Japanese compound. The ~ている marks ongoing employment.