Law enforcement in protected areas is critical for ensuring long-term conservation and achieving conservation objectives. In 2004, patrol-based monitoring of law enforcement was implemented in protected areas in Ghana. Here, we evaluate long-term trends and changes in patrol staff performance, and illegal activities, in the Kogyae Strict Nature Reserve. The assessment was based on ranger patrol-based monitoring data collected during January 2006–August 2017. Along the patrol routes, patrol officers recorded all encounters with illegal activities associated with hunting and capturing or harming of animals. Across all years, staff performance was lowest in 2006 as staff learned the system but increased in 2007 and peaked in 2010, the latter as a result of motivation of the patrol staff. After 2011, staff performance decreased, mainly because of the retirement of some patrol staff and insufficient logistical support for successful patrolling. Snares were the most commonly recorded indicators of illegal activity. Because their use is silent, poachers using snares are less likely to be detected than poachers using other forms of hunting. Long-term assessment of patrol-based monitoring data provides reliable information on illegal activities related to wildlife, to enable stakeholders to design effective measures for biodiversity conservation. Our assessment indicates that patrol staff performance in Kogyae is, at least partly, dependent on governmental or external support and incentives, in particular the provision of equipment and transport facilities.