The death rate from cancer and infectious diseases is still very high, therefore research in this area is extremely important and promising as in medical, so in economic point of view. Thus, potassium salt of tetrazolo[1,5-c]quinazolin-5-thion was modified per alkylation by N-aryl(benzyl,heteryl)acetamides with proper confirmation of newly synthesized compounds’ structures by FT-IR, LC–MS, 1H NMR and elemental analysis data. The substances were tested for bioluminescence inhibition against Photobacterium leiognathi Sh1 (5–50 μg/mL) to check their cytotoxicity. Then they were screened for antibacterial and antifungal activities (100 μg) against Escherichia coli, Staphylococcus aureus, Enterobacter aerogenes and Enterococcus faecalis, Pseudomonas aeruginosa, Klebsiella pneumoniae and Candida albicans. It was found that compounds 1.1, 1.5, 1.10, 1.31, 1.33 possessed light activity against K. pneumonia. The US National Cancer Institute (NCI) has chosen 19 compounds and screened them for ability to inhibit in 10 μM concentration 60 different human tumor cell lines. The LOX IMVI cell line of melanoma appeared to be the most sensitive one, and N-(6-methylbenzo[d]thiazol-2-yl)-2-(tetrazolo[1,5-c]quinazolin-5-ylthio)acetamide (1.31) and N-(3-fluorobenzyl)-2-(tetrazolo[1,5-c]quinazolin-5-ylthio)acetamide (1.19) exhibited high growth inhibition rate, and N-(6-methoxybenzo[d]thiazol-2-yl)-2-(tetrazolo[1,5-c]quinazolin-5-ylthio)acetamide (1.32) showed lethal antitumor activity against it. The latter compound 1.32 showed the best anticancer results, also inhibiting growth of leukemia SR cell line, NCI-H460 of non-small cell lung cancer, KM12 of colon cancer and SF-295 of CNS cancer. The in silico molecular docking studies have predicted the affinity of the synthesized substances to the epidermal growth factor receptor (EGFR).