A monolithic silicon pixel prototype produced for theMONOLITH ERC Advanced project was irradiated with 70 MeV protons upto a fluence of 1 × 10$^{16}$1 MeV n$_{eq}$/cm$^{2}$. The ASIC contains a matrix ofhexagonal pixels with 100 μm pitch, readout by low-noise andvery fast SiGe HBT frontend electronics. Wafers with 50 μmthick epilayer with a resistivity of 350 Ωcm were used toproduce a fully depleted sensor. Laboratory tests conducted with a$^{90}$Sr source show that the detector works satisfactorily afterirradiation. The signal-to-noise ratio is not seen to change up tofluence of 6 × 10$^{14}$n$_{eq}$/cm$^{2}$. The signaltime jitter was estimated as the ratio between the voltage noise andthe signal slope at threshold. At -35°C, sensor bias voltageof 200 V and frontend power consumption of 0.9 W/cm$^{2}$, the timejitter of the most-probable signal amplitude was estimated to be σ$_{t}$$^{90}$Sr = 21 ps for proton fluence up to6 × 10$^{14}$n$_{eq}$/cm$^{2}$ and 57 ps at1 × 10$^{16}$n$_{eq}$/cm$^{2}$. Increasing the sensorbias to 250 V and the analog voltage of the preamplifier from 1.8to 2.0 V provides a time jitter of 40 ps at1 × 10$^{16}$n$_{eq}$/cm$^{2}$.