Global change reshapes arthropod communities, yet long‐term effects on their functional diversity remain unclear. Using a long‐term dataset comprising > 1.3 million individuals of 728 carabid and spider species, we quantified temporal trends in functional diversity across forests, grasslands and croplands. Trajectories diverged strongly among land uses: functional richness increased in forests and grasslands but declined sharply in croplands. In forests, communities became more specialised and less redundant, suggesting increasing niche differentiation, but experienced greater functional richness loss under simulated species extinctions. Cropland assemblages became functionally homogenised. Losses of key effect traits among spiders suggest potential erosion of ecosystem functions in croplands. Moreover, genera dominated by species with unique trait combinations were typically larger‐bodied and less dispersive, and their loss would remove irreplaceable functional strategies from communities. Overall, our study reveals diverging long‐term trajectories of arthropod functional diversity across land uses, indicating potential negative consequences for ecosystem functioning in arable landscapes.