ObjectiveTo determine the effects of early entry into the labour market and physicality of work in young adulthood on the development of obesity and unhealthy lifestyle habits later in life.MethodsThis study is a part of the Young Finns Study. Entry into the labour market and physicality of work were measured at baseline, when participants were aged 18, 21, or 24 years in 1986 or 18 years in 1989. Follow-up of lifestyle habits were conducted in 2001, 2007 and 2011. The outcomes were obesity (n=5558 observations), abdominal obesity (n=4060 observations), daily smoking (n=5628) and leisure time physical activity (n=5946) and analysed with generalised estimating equation.ResultsCompared with sedentary work, physicality of work in young adulthood increased the odds of future obesity (adjusted OR=1.32, 95% CI 1.01 to 1.74 for light/moderate work and OR=1.44, 95% CI 0.99 to 2.08 for heavy manual work (particularly in women OR=2.03, 95% CI 1.07 to 3.84)) and future smoking (OR=1.79, 95% CI 1.39 to 2.30 for light/moderate work and OR=2.01, 95% CI 1.47 to 2.76 for heavy manual work (particularly in women OR=2.81, 95% CI 1.60 to 4.91)). For those who entered the labour market at ages 18–21 or younger, the odds of smoking was 1.85 times (95% CI 1.26 to 2.73) and that of obesity 1.45 times (95% CI 1.01 to 2.10) higher, and the rate of leisure time physical activity was 0.73 times (95% CI 0.58 to 0.93) lower compared with those who entered the labour market at ages 22–24 years.ConclusionEarly entry into the labour market and physicality of work in young adulthood shape the development of obesity and unhealthy behaviours in later adulthood.