An increased mortality risk was found among men aged between 37 and 49, the age group most affected in the University of Glasgow study.
The study, published in BMC Medicine, also indicated absolute mortality remains higher in the 60 to 73 age group.
The effect of having two or more long-term health conditions is associated with a higher risk of death from cancer, vascular conditions and all causes of death even after accounting for lifestyle or demographic factors.
READ MORE: Ten Scots a week facing amputation as obesity soarsOver the median seven years of follow-up, those with four or more conditions were nearly three times more likely to die early compared to those with no conditions.
Professor Frances Mair, at the University of Glasgow, said: "This study is significant because it presents novel findings regarding what we know about multimorbidity.
"This is the first study to examine the relationship of multimorbidity with cancer mortality and we have shown a dose-response relationship between number of long-term conditions and cancer mortality."
Research also indicated the type of long-term condition, rather than the number of them, could play an important role.
Dr Bhautesh Jani was also lead author of the study - Relationship Between Multimorbidity, Demographic Factors and Mortality: Findings from the UK Biobank Cohort.
He said: "Younger participants, especially men, were observed to have a relatively higher risk of mortality with increasing number of long-term conditions, and that certain combinations of conditions were associated with a particularly higher risk of death."Going forward, further research is needed to study the impact and management of multimorbidity in middle-aged adults as they may be at higher risk of early death."