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She held the female office workers she'd been working with up until then in contempt.
Literal
She [topic-は] [up to that point-それまで] together work [object-を] doing-came female-office-workers [object-を] held-in-contempt.
それまで ('up until that point, up until then') anchors the time frame relative to a moment already established in context. 仕事をしてきた ('had been working,' literally 'work + has come') uses ~てきた — the past form of ~てくる ('do up to now / continue toward present') — to mark the working as a continuing-up-to-then activity. The whole long phrase from それまで to 達 is a relative clause modifying 女性事務員達 ('female office workers'). 軽蔑する ('despise, hold in contempt') is a strongly negative verb — heavier than 嫌う ('dislike') and more pointed than 見下す ('look down on'). The sentence has a slightly formal, almost essayistic feel.