Pada Juli 2018, Katie bergabung dengan Manchester United W.F.C musim 2018-19 di ajang [[Kejuaraan Wanita FA]].<ref>{{Cite web|url=https://www.manutd.com/en/news/detail/official-announcement-of-players-in-manchester-united-women-squad|title=Man United Women's 2018/19 squad|website=www.manutd.com|language=en|access-date=2020-07-28}}</ref> Ia tampil pertama kali, saat kemenangan melawan Liverpool L.F.C 1 - 0 dalam ajang Piala Liga Wanita FA pada 19 Agustus.<ref>{{Cite web|url=https://www.manutd.com/en/news/detail/match-report-liverpool-women-0-man-utd-women-1|title=Match report Liverpool Women v Man Utd Women|website=www.manutd.com|language=en|access-date=2020-07-28}}</ref> Katie mencetak gol pertamanya pada musim 2018-19 saat melawan Aston Villa dalam tendangan pinalti dan membantu membawa timnyatim nya menang 12 - 0.<ref>{{Cite web|url=https://www.manutd.com/en/news/detail/match-report-aston-villa-v-united-women-on-9-september-2018|title=Match report Aston Villa v United Women on 9 September 2018|website=www.manutd.com|language=en|access-date=2020-07-28}}</ref> Katie menjadi pemain terbaik pada bulan Maret 2019.<ref>{{Cite web|url=https://www.manutd.com/en/news/detail/katie-zelem-wins-fa-womens-championship-player-of-the-month-award-for-march-2019|title=Zelem wins March POTM prize|website=www.manutd.com|language=en|access-date=2020-07-28}}</ref> Pada akhir musim, ia dinobatkan sebagai pemain terbaik Manchester United W.F.C.<ref>{{Cite web|url=https://www.manutd.com/en/news/detail/katie-zelem-wins-united-women-player-of-the-year-award-2018-19|title=Zelem is United Women's Player of the Year|website=www.manutd.com|language=en|access-date=2020-07-28}}</ref> Pada musim 2019-20, Katie terpilih sebagai kapten Manchester United W.F.C.<ref>{{Cite Padaweb|url=https://www.manutd.com/en/news/detail/man-utd-women-announce-new-captain-ahead-of-the-201920-season|title=Katie 28Zelem Septembernamed 2019,as iaMan membantuUtd timnya menang 2Women captain|website=www.manutd.com|language=en|access-date=2020-07-28}}</ref> 0 melawan Liverpool L.F.C.
