Activated Cdc42-associated kinase (ACK) is a Rho family effector that is widely implicated in cancer, yet not currently pharmacologically inhibited in the clinic. Through relationships with the androgen receptor in prostate cancer and the oestrogen receptor in breast cancer, ACK regulates transcriptional networks in disease. In this work, new roles for ACK in transcription, mediated by its relationship with the signal transducer and activators of transcription (STAT) family, are uncovered. The STAT family are a 7-member family of transcription factors that are hyperactivated in disease, yet no STAT inhibitors have reached clinical approval and thus, understanding of ACK-driven STAT activation is of huge therapeutic interest. In this work, ACK is shown to interact with STAT family members and augment their phosphorylation at the conserved activation tyrosine residue. This activation event is shown to drive STAT3, STAT5A and STAT5B nuclear translocation and transcriptional activation. Thus confirming ACK as a STAT activator. The ACK-STAT relationship is also demonstrated at an endogenous level, with a specificity to haematological disease cell lines. In the K562 chronic myeloid leukaemia cell line, ACK is identified to interact with STAT5 and by cell line engineering was confirmed to contribute to the nuclear pool of transcriptionally active STAT5. By thorough bioinformatic analysis, the true contribution of ACK as a mediator of STAT5 transcription was uncovered. Thirty one ACK- STAT5 transcriptional targets were identified in K562s, the first known ACK-driven transcriptional signature in a disease type. ACK loss in K562s was also phenotypically challenging, with proliferation, epithelial-mesenchymal transition, DNA damage response, glycosylation, adhesion, F-actin polymerisation and drug sensitivity all being altered upon ACK loss. Therefore, this work confirms ACK as a contributor to hyperactivated STAT5 signalling in a CML cell line and reveals a new route for therapeutic intervention.