Inconel 718 is a nickel-iron based superalloy widely used in the aerospace industry. Its high temperature strength is attributed to the thermal stability of dense nanoscale precipitates ${\ensuremath{\gamma}}^{\ensuremath{'}}$ $[{\mathrm{Ni}}_{3}\mathrm{Al}]$ and ${\ensuremath{\gamma}}^{\ensuremath{''}}$ $[{\mathrm{Ni}}_{3}\mathrm{Nb}]$. There is experimental evidence that ${\ensuremath{\gamma}}^{\ensuremath{'}}$ and ${\ensuremath{\gamma}}^{\ensuremath{''}}$ often form co-precipitates ${\ensuremath{\gamma}}^{\ensuremath{'}}∕{\ensuremath{\gamma}}^{\ensuremath{''}}$ or sandwichlike structure ${\ensuremath{\gamma}}^{\ensuremath{'}}∕{\ensuremath{\gamma}}^{\ensuremath{''}}∕{\ensuremath{\gamma}}^{\ensuremath{'}}$ or ${\ensuremath{\gamma}}^{\ensuremath{''}}∕{\ensuremath{\gamma}}^{\ensuremath{'}}∕{\ensuremath{\gamma}}^{\ensuremath{''}}$. But how they stabilize under heat treatment or in service is still not well-understood. We have investigated the interfacial structure and chemistry of fine co-precipitates ${\mathrm{Ni}}_{3}(\mathrm{Al},\mathrm{Ti},\mathrm{Nb})∕{\mathrm{Ni}}_{3}\mathrm{Nb}({\ensuremath{\gamma}}^{\ensuremath{'}}∕{\ensuremath{\gamma}}^{\ensuremath{''}})$ in Inconel 718, using both first-principles density functional theory calculation and the three-dimensional atom probe technique. Our calculations confirm that Al atoms in the ${\ensuremath{\gamma}}^{\ensuremath{'}}$ phase segregate to the ${\ensuremath{\gamma}}^{\ensuremath{'}}∕{\ensuremath{\gamma}}^{\ensuremath{''}}$ interface. The enrichment of Al helps to impede the assimilation of Nb from ${\ensuremath{\gamma}}^{\ensuremath{'}}$ to ${\ensuremath{\gamma}}^{\ensuremath{''}}$ and reject Al from ${\ensuremath{\gamma}}^{\ensuremath{''}}$ to ${\ensuremath{\gamma}}^{\ensuremath{'}}$, and therefore keeps such secondary precipitates at fine size. In the absence of Ti in the ${\ensuremath{\gamma}}^{\ensuremath{'}}$ phase, our calculations predict an enhanced driving force for Al to accumulate at the interface. We have also characterized the microstructure of the ${\ensuremath{\gamma}}^{\ensuremath{'}}∕{\ensuremath{\gamma}}^{\ensuremath{''}}$ interface for an Inconel 718 sample taken from a commercial compressor blade serviced in an airplane engine for over $10\phantom{\rule{0.2em}{0ex}}000\phantom{\rule{0.3em}{0ex}}\mathrm{h}$ at a temperature up to $600\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ using three-dimensional atom probe analysis. Interestingly, we find that Al enrichment sustains long-term service, suggesting that the coarsening of secondary precipitates is interface-controlled. The success of first-principles density functional theory computation in reproducing the experimental observation encourages extensive application of this powerful tool in the study of precipitates in superalloys.