With outstanding analytical performances, mass spectrometry (MS) has shown great potential for clinical applications. To facilitate the sampling process and quantitative analysis, a capillary-in-capillary electrospray ionization (CC-ESI) source was developed in this study. Utilizing two nested capillaries as a sampler and an ESI emitter, the source enabled spontaneous liquid sampling based on the capillary phenomenon and electrospray ionization mass spectrometry (ESI-MS) analysis. Apart from the cheap price, high portability, and disposability, the CC-ESI had merits of quantitation capability as well as adequate sensitivity. By coupling CC-ESI to a miniature mass spectrometer (mini-MS), a limit of detection (LOD) of 1 ng/mL was achieved for standard imatinib at collision-induced dissociation (CID) tandem MS mode, and a LOQ of 1 ng/mL was obtained for atenolol and imatinib (with isotopic internal standard) at multiple ion reaction monitoring (MRM) modes. As two demonstrations for analysis of practical samples, rapid analysis of abused drugs on surface and quantitative analysis of therapeutic drugs in whole blood were also performed with a CC-ESI mini-MS.