We show for the first time that the high molecular weight fractions of kraft lignin can be depolymerised by a mechanochemical one-pot reaction with sodium percarbonate (SPC) and all the lignin can be functionalised simultaneously. SPC is an excellent choice because it is already well established in the industry and is an environmentally friendly alternative to conventional oxidants, as the degradation products of H2O2 are harmless to the environment. By means of IR it was shown that SPC in combination with NaOH has an increased oxidising capacity, which leads to an increased carbonyl functionality in the depolymerised lignin. The GPC data show that the greatest degradation effect in lignin occurs after 5 minutes of reaction time, after which the lignin oligomers recombine. The recombination can be reduced by the addition of NaOH. Using DVS, it was shown that an increase in carbonyl functionalities leads to a significant water adsorption by a factor of ~2.8. We hypothesise that the increase in carbonyl functionalities causes the increase in water adsorption. This hypothesis is supported by SAXS data showing that the surface area of oxidized lignin does not increase significantly, suggesting that the increased water adsorption is mainly due to chemical functionalization. The reduction in polydispersity as well as the improved water adsorption could make lignin a suitable candidate for the development of sustainable water-absorbing polymers.