We use the SDSS Stripe 82 to quantify the role of minor mergers in driving stellar-mass and black hole growth in the local Universe. Since major mergers destroy discs and create spheroids, morphologically disturbed spirals are remnants of minor mergers. Disturbed spirals exhibit enhanced specific star formation rates (SSFRs), the enhancement increasing in galaxies of `later' morphological type (which have more gas and smaller bulges). By combining the SSFR enhancements with the fraction of time spirals spend in this `enhanced' mode, we estimate that ~40 per cent of the star formation in local spirals is directly triggered by minor mergers. The disturbed spirals also exhibit higher nuclear-accretion rates, implying that minor mergers enhance the growth rate of the central black hole. However, the specific accretion rate shows a lower enhancement than that in the SSFR, suggesting that the coupling between stellar-mass and black-hole growth is weak in minor-merger-driven episodes. Combining our results with the star formation in early-type galaxies - which is minor-merger-driven and accounts for ~14 per cent of the star formation budget - suggests that around half of the star formation activity in the local Universe is triggered by the minor-merger process. Without a deeper comprehension of this key process, our understanding of galaxy evolution will remain incomplete.