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Factors from the domains of work, social support networks and activity participation were examined in an ageing group retrospectively and contemporaneously. Aged 11, these individuals had taken a test of mental ability (the Moray House Test: MHT) as part of the 1932 Scottish Mental Survey. Some 550 survivors were recruited ~ 79 years old into a longitudinal study of cognitive ageing – the Lothian Birth Cohort 1921 – when they again took the MHT, plus a battery of tests (Raven’s Progressive Matrices, Verbal Fluency and Logical Memory). A 2nd wave of testing was completed at ~83 years old. Over the 4 years of follow-up, significant decline was observed in cognitive ability (composite of the 3 tests) and separately for Raven’s and Verbal Fluency, but not for Logical Memory. In regression analyses, higher cognitive ability at age 79 assessed by the MHT (expressed as age-79 IQ) was predicted by less hazardous working conditions, a quieter working environment and receiving more supervisor support; living with a spouse/partner for a fewer number of years in young adulthood and having fewer close friends/relatives in old age; and increased activity in midlife and old age. Each factor explained about 1% to 3% of the variance, independent of age-11 IQ and sex. Less cognitive decline from 79 to 83 years old was associated with increased support from coworkers and walking (versus not) at age 80, each accounting for about 2% of the variance (independent of age-11 IQ and sex). When the analyses were pooled across lifestyle domains and further potential confounders were controlled, measures of the hazards encountered at work and lifetime activity participation each accounted for 1% to 3% of the variance in age-79 IQ, whilst walking accounted for about 2% of the variance in later life cognitive change. Inactivity in midlife and a lack of exercise in old age are plausible risk factors for cognitive decline.