Since the 1991 enactment of mandatory helmet legislation (MHL) for cyclists in New South Wales (NSW), Australia, there has been extensive debate as to its effect on head injury rates at a population level. Many previous studies have focused on the impact of MHL around the time of enactment, while little has been done to examine the ongoing effects. We aimed to extend prior work by investigating long-term trends in cyclist head and arm injuries over the period 1991–2010. The counts of cyclists hospitalised with head or arm injuries were jointly modelled with log-linear regression. The simultaneous modelling of related injury mechanisms avoids the need for actual exposure data and accounts for the effects of changes in the cycling environment, cycling behaviour and general safety improvements. Models were run separately with population counts, bicycle imports, the average weekday counts of cyclists in Sydney CBD and cycling estimates from survey data as proxy exposures. Overall, arm injuries were higher than head injuries throughout the study period, consistent with previous post-MHL observations. The trends in the two injury groups also significantly diverged, such that the gap between rates increased with time. The results suggest that the initial observed benefit of MHL has been maintained over the ensuing decades. There is a notable additional safety benefit after 2006 that is associated with an increase in cycling infrastructure spending. This implies that the effect of MHL is ongoing and progress in cycling safety in NSW has and will continue to benefit from focusing on broader issues such as increasing cycling infrastructure.