Antimicrobial use in companion animals is a largely overlooked contributor to the complex problem of antimicrobial resistance. Humans and companion animals share living spaces and some classes of antimicrobials, including those categorised as Highest Priority Critically Important Antimicrobials (HPCIAs). Veterinary guidelines recommend that these agents are not used as routine first line treatment and their frequent deployment could offer a surrogate measure of 'inappropriate' antimicrobial use. Anthropological methods provide a complementary means to understand how medicines use makes sense 'on-the-ground' and situated in the broader social context. This mixed-methods study sought to investigate antimicrobial use in companion animals whilst considering the organisational context in which increasing numbers of veterinarians work. Its aims were to i) to epidemiologically analyse the variation in the percentage of antimicrobial events comprising of HPCIAs in companion animal dogs attending UK clinics belonging to large veterinary groups and, ii) to analyse how the organisational structure of companion animal practice influences antimicrobial use, based on insight gained from anthropological fieldwork. A VetCompassTM dataset composed of 468,665 antimicrobial dispensing events in 240,998 dogs from June 2012 to June 2014 was analysed. A hierarchical model for HPCIA usage was built using a backwards elimination approach with clinic and dog identity numbers included as random effects, whilst veterinary group, age quartile, breed and clinic region were included as fixed effects. The largest odds ratio of an antimicrobial event comprising of a HPCIA by veterinary group was 7.34 (95% confidence interval 5.14 - 10.49), compared to the lowest group (p < 0.001). Intraclass correlation was more strongly clustered at dog (0.710, 95% confidence interval 0.701 - 0.719) than clinic level (0.089, 95% confidence interval 0.076 -0.104). This suggests that veterinarians working in the same clinic do not automatically share ways of working with antimicrobials. Fieldwork revealed how the structure of the companion animal veterinary sector was more fluid than that depicted in the statistical model, and identified opportunities and challenges regarding altering antimicrobial use. These findings were organised into the following themes: "Highest priority what?"; "He's just not himself"; "Oh no - here comes the antibiotics police"; "We're like ships that pass in the night"; and "There's not enough hours in the day". This rigorous mixed-methods study demonstrates the importance of working across disciplinary silos when tackling the complex problem of antimicrobial resistance. The findings can help inform the design of sustainable stewardship schemes for the companion animal veterinary sector.