Mortality rates have been falling or ‘improving’ in many demographically developed countries since the 1950s. However, there has been a slowdown since 2010 in the speed of improvement and this phenomenon has been particularly marked at ages over 50. To understand better this mortality slowdown, we have analysed long-run mortality trends of a group of developed countries using data up to 2017 from the Human Mortality Database. Specifically, we have used statistical models to parametrise the historical mortality trends of 21 countries between 1965 and 2010 and then forecast trends beyond 2011. We find that many countries have experienced lower mortality improvement rates in 2011–2017 than in the previous decade and also experienced lower improvement rates in 2011–2017 than would have been forecast based on the models fitted to data prior to 2011. Some of the Scandinavian populations have bucked the stalling mortality improvement trend, experiencing higher mortality improvement rates than the forecasts. We conclude that part of the slowdown in mortality improvement rates of the over 1950s since 2011 would have been expected from historical trends in many countries, especially among men. However, there has been a notable slowdown since 2011, compared with the model forecasts, in many countries especially among women. A few countries had higher mortality improvement rates than forecast. A better understanding of the drivers behind these complex trends would help decision makers in insurance companies and pension funds and also inform public policy.