The mechanisms of coalescence and T-junction formation of carbon nanotubes are analyzed using action-derived molecular dynamics. The control of kinetic energy in addition to the total energy leads to the determination of the minimum-energy atomistic pathway for each of these processes. Particularly, we find that the unit merging process of two carbon nanotubes consists of four sequential generalized Stone-Wales transformations occurring in four hexagon-heptagon pairs around the jointed part. In addition, we show that a single carbon atom may play the role of an autocatalyst, which significantly reduces the global activation energy barrier of the merging process. For T junction formation, two different models are chosen for simulation. One contains defects near the point of junction formation, while the other consists of two perfect nanotubes plus two additional carbon atoms. Our results indicate that the coalescence and junction formation of nanotubes may occur more easily than theoretically predicted in the presence of additional carbon atoms at moderate temperatures.